1. Field of the Invention
This invention pertains to thread garuges for the measurement of and/or the verification inspection of the threads on the pin ends and box ends of tubular members and more specifically to the manufacturing of a suitable gauge for that purpose when the conformation of the threads are proprietary or unknown for any reason.
2. Description of the Prior Art
Pipe sections or joints, casings, tubings, or other tubular members used in the oil and gas industry or otherwise normally include a pin end and a box end, the threads of which are intended to be threaded to be complementary and are manufactured in accordance with specified standards to provide secure joints. The most important characteristics of a joint connection that determines that there is a good seal when joints are made up are (1) that the threads mate so as to engage uniformly, and (2) that the threads mate at a sufficient depth so that adverse stresses in making up joints and using such joints are avoided.
The following discussion assumes that it has been previously determined that the lead on the screw threads are uniform. That is, the threads are within tolerances so that there are the correct and same number of threads per inch for each pin end and box end in the plurality of tubular members to be verified and classified in accordance with the invention hereinafter described. Such thread measurement is readily accomplished by a simple gauge that fits into the threads and makes this determination.
However, this is not a sufficient verification since the taper and relative sizes of mating pin end and box end can be such to cause high stresses and other problems. If the box end taper is correct, but the pin end taper is "slow" (too gradual), then the two ends tend to cause a pressure point near the tip of the pin end. This could cause compressional yielding and eventual pin end failure. On the other hand, if the pin end taper is "fast" (too steep), then the peak pressure point can be too near the outer rim of the box end, thereby possibly causing hoop tensile failure of the box end. Of course, a correct pin end taper meeting with a slow or fast box end taper would have correspondingly similar results.
Mating threads of slightly different pitch diameter present another problem. If the pin end threads have an oversized pitch diameter with respect to the box end threads, then the pin end will not completely make up in the box end at normal power make up pressure. Forcing a further make up will cause either or both metal breakage at the crests or gouging at the roots of the mating threads. Furthermore, if the pitch diameters are sufficiently mismatched, material failure of either or both the pin end and box end could occur. If the pin end threads are undersized in pitch diameter with respect to the box end threads, then an insufficient metal-to-metal seal is effected, resulting in possible leakage around the threads and/or a failure (separation) to the thread crests since only a shallow portion of the threads carry the load,
Although there are many standard threads with designated tapers, pitches, pitch diameters, clearances, allowances and the like, there are also proprietary threads in service. The quality standard do not always screen out tubular members that have threads which are out of tolerance in one or more of the aforementioned areas. Although it is possible for a distributor or user of the tubular members to find gauges for verifying "standard" threads, such gauges available in the marketplace tend to be expensive, not readily available and potentially inconvenient to use. Of course, there may not be any gauges available at all for "proprietary" threads. Furthermore, even if it is assumed that all of the tubular members to be used at a job are within individual tolerance, it is readily apparent that a larger number of better connections can be made by ascertaining the taper conditions and the pitch diameters and then selectively matching the pin ends with the box ends. Such matching also avoids a connection of two marginal mating parts that are on the edge of the tolerance limits in opposite directions.
It is very important at the mill or at the yard to which tubular members are delivered or even in the field where the tubular members are used to be able to verify that the threads are suitably within tolerance to mate with one another so that consecutive joints make up completely and that a thorough metal-to-metal seal had been accomplished. Although sophisticated gauges are available at the mill to employ during the machining operation, heretofore there has not been a simple, yet sufficiently accurate, method of assuring that the threads of both the pin ends and of the box ends of a plurality of supposedly "identical" sections of tubular members are of such uniformity that satisfactory junctions are obtained when these members are randomly joined together.
Therefore, it is a feature of the present invention to provide an improved gauge for use on a plurality of tubular members for verifying their acceptability and maximizing their relative fit, even when the thread conformation is unknown.
It is another feature of the present invention to produce an improved gauge for such purpose from a known tubular member section whose threads are within the acceptable limits established for the plurality of tubular members.
It is yet another feature of the present invention to produce improved ring and pin gauges for determining "seal diameter" and "standoff" dimensions for a plurality of tubular members, which gauges are readily used in the field.
It is still another feature of the present invention to produce improved ring taper and plug taper gauges for determining pin end taper and box end taper for a plurality of tubular members, which gauges are readily used in the field.